Abstract

A spectroscopic survey has been performed of 69 stars on or near the
giant branches of the metal-poor globular cluster NGC 6752. Our basic
results are: (i) There is a large range in the strength of the violet
cyanogen bands on the red giant branch, with the available evidence
strongly suggesting that the distribution is bimodal. (ii) The cyanogen
variations on the giant branch appear to be accompanied by an
anticorrelated variation in the abundance of the CH molecule. Spectrum
synthesis analysis of a (CN strong)/(CN weak) pair of stars for which
relatively high resolution data are available shows that there is a
variation of Delta[N/A] ˜+0.9, and Delta[C/A] ˜-0.3,
indicative of the CN cycle. (iii) On the red giant branch there are
variations in the strength of the lines of Al I which correlate
positively with the cyanogen variations. The size of the variations is
consistent with the hypothesis that the same phenomenon has occurred in
NGC 6752 and omega Centauri, but to a much smaller extent in the
former. (iv) On the asymptotic giant branch (AGB), the features of CH
are weaker than on the red giant branch at the same color or magnitude,
and there are no examples of stars in the strong CN group. Spectrum
synthesis suggests that the behavior of the CH features is consistent,
on the average, with the effective temperature and gravities of the AGB
stars, but that the absence of strong CN stars cannot be explained in
this way. We set an upper limit of Delta[C/H] ˜0.3 to the
possible range of carbon on the AGB at log L/L -- stars: individual:
˜2.3, and between this group and stars of similar color on the red
giant branch. (v) Most of the stars on the anomalously low luminosity
end of the AGB are not members of NGC 6752. Two stars, (CS 41 and CS
44), however, deserve further study, since they could be examples of
partially mixed stars.

No definitive statement can be made concerning the origin of the
abundance anomalies. if mixing is responsible, the data require this
process to have operated in stars on the giant branch at a luminosity of
log L/Lsun ˜1.6 (the level of the horizontal branch),
and that at least half the stars have mixed. The Al I variations are
difficult to explain as the result of mixing. if, on the other hand, the
variations are primordial, it is difficult to envisage a succession of
stellar generations which can lead to the two observed, roughly equal,
populations in which the nitrogen strong, Al I strong group is deficient
in carbon by a factor of roughly 2. One somewhat ad hoc primordial model
which is consistent with the observations is that proton NGC 6752 was
made up of two cells of different chemical histories which merged to
leave the cluster as we see it today.

We consider two hypotheses which are consistent with the present
observations and the existence of the well known gap on the horizontal
branch of NGC 6752. in the first we suggest that, when star formation
ceased in the cluster, there were two groups of stars having not only
the observed carbon and nitrogen properties, but also a difference in
helium abundance, DeltaY ˜0.05, in the sense that the nitrogen
strong group has enhanced helium. This difference in helium leads to a
mass difference of ˜0.07 Msun at the main sequence
turnoff, which, together with our current knowledge of horizontal branch
morphology, provides an explanation of both the gap on the horizontal
branch and the lack of CN strong stars on the AGB. (The high helium,
high CN group does not ascend the giant branch for a second time.) The
second hypothesis supposes that mixing is responsible for the abundance
anomalies, and that this process is associated with greater mass loss,
leading to two mass groups on the horizontal branch. (Here, too, the CN
strong, low mass group does not ascend the giant branch for a second
time.)

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